A biocular eyepiece is an optical system which may be viewed by an observer with both eyes. The observer may look through different parts of the same optical system without any discontinuity. The device has been widely accepted as a means to magnify the image of a cathode ray tube (CRT), an image intensifier, or a forward looking infrared system (FLIR). It has also been widely used in head-up and head-down displays for both modern military and commercial aircraft, flight simulators, microscopes for semiconductor device inspection, and medical applications. The chief advantage of using a biocular eyepiece, as compared to a monocular eyepiece or a binocular eyepiece, is that the observer is able to freely move his head and use both eyes to see essentially the same image at the same light level on the same optical system.
Classical biocular eyepieces such as those disclosed in U.S. Pat. Nos. 3,922,072 and 4,183,624 require at least five to six refractive optical elements and generally have an overall length about twice that of the effective focal length. In some applications the overall length of these designs can lead to problems. For example, a desirable feature of equipment used in aircraft is a minimal space requirement such that more equipment may be used in a limited space. The fact that optical systems require five to six optical elements make conventional optical systems relatively heavy. In addition, the cost of a biocular system increases with the number of lens elements. The known biocular eyepiece design consists of several focusing lenses and at least one negative optical power element for chromatic aberration correction and field curvature reduction. When a biocular eyepiece of the conventional design deviates from the stated design conditions, the chromatic aberration, higher order aberration, field curvature, and distortion will be aggravated.
At least one attempt to reduce the number of elements in a conventional biocular eyepiece employed a curved CRT fiber faceplate to reduce the field curvature. A reduced spectral band emission minimized the chromatic aberration. Unfortunately, an unconventional CRT adds a significant amount of cost to the system. More importantly, the operating voltage of the CRT usually needs to be increased to compensate the loss of brightness associated with this design. The lifetime of the CRT is therefore reduced. Presently, it is generally recognized that it is a poor substitute to curve and distort the object in order to compensate for the deficiencies of the optical system.
What is needed is a simple biocular eyepiece which yields a performance that is comparable or better than conventional eyepieces, but with fewer optical elements contained in a more compact size. It is an object of the preferred embodiment of this invention to provide such an apparatus.